1. Principle of Screw and Plate Fixation & Mechanical Behavior of Implant Materials

2. Roles of Implants Add stability Replace damaged or diseased part
Fracture fixation
A plate used after osteotomy
Replace damaged or diseased part
Total joint replacement
Healing stimulants

3. Advantages of Internal Fixation
No casts
Prevent skin pressure and fracture blisters
No scars
No complications of bed rest
Important for the elderly
Early motion
Avoid stiffness
Enhance fracture healing
Prevent muscle atrophy

4. Principles of Fixation
Rigid fixation
Stress distribution
Fracture stability
Compression
Stability
Primary healing
Membranous bone repair

5. Biomechanics of Dynamic Compression Plate (DCP)
Designed to compress the fracture
Offset screws exert force on specially designed holes in plate
Force between screw and plate moves bone until screw sits properly
Compressive forces are transmitted across the fracture
Beginning
End Result
ttb.eng.wayne.edu/ ~grimm/ME518/L19F3.html

6. DCP (Cont’d) Alternate embodiment:
External compression screw control
Additional pictures of internal plate

7. Plate Placement Lateral cortex Flexural rigidity
E * I
Depends on direction of loading
Area moment of inertia

8. Plate and oblique fracture
A: For ONLY torsional loads: 45° to long axis
B: For ONLY bending loads: Parallel to long axis
Realistically: loads in both directions will be applied: Divide angle between long axis and 45° B

9. Dynamic Hip & Condular Screw Indications (DHS)
Fractures of the proximal femur
Intertrochanteric fractures
Subtrochanteric fractures
Basilar neck fractures
Stable fractures
Unstable fractures in which a stable medial buttress can be reconstructed
Provide controlled collapse and compression of fracture fragments

10. Sliding Compression Screw Devices
Screw in center of femoral head (proximal fragment)
Slides through barrel attached to plate
See yellow arrow
Essential to obtain max hold capacity in head of femur
Plate is attached to bone (distal fragment) by screws
Screw threads designed to allow optimum fracture compression and hold

11. Sliding Screw Plate Angle
For anatomic reduction
Less force working across sliding axis than higher angle plates
Prevents impaction
Used effectively in stable fractures
Controlled collapse is not important

12. Sliding Screw Plate Angle
For unstable fractures
Mechanically, it is desirable to place sliding device at as high angle as clinically possible while still maintaining placement of device in center of head
Technically surgeon cannot place sliding device at high angle in small hip or in hip with varus deformity

13. DHS Technique Incisional line
Red = conventional
Green = minimal access
Procedure is monitored by x-ray image intensifier

14. DHS Targeting Device Aligns guide pin Under the vastus lateralis
Wedged in upper part
Between vastus and femoral shaft

15. DHS Guide Pin Guide pin is inserted Centered in the femoral neck

16. DHS Axial Screw Axial screw is inserted with an extension
Extension to guide the barrel of plate
Slot along screw fits a longitudinal ridge inside barrel prevents rotation, allows axial compression only

17. DHS Plate Plate against femoral shaft Shaft screws are inserted

18. DHS Problems With the plate attached to the bone
Bone below the plate is at an increased risk of a stress fracture
Quality of bone is important
Procedure will vary among patients with healthy or osteoporotic bone

19. Metal: Rough & Polished
Materials
Composite
Metal: Rough & Polished
Polymer
Ceramic

20. Bio Materials Synthetic materials Non viable material
Interacts with biological systems
Corrosion
Debris
To augment or replace tissues and their functions

21. Types of materials Metals Composites Polymers Ceramics
Polyethylene (PE)
Silicone
Ceramics
Bone cement (PMMA)
Biodegradable

22. Metals Titanium Cobalt-chromium-molybdenum Stainless Steel
Chemical Make-up
Ti6Al4V
30-60% Co
20-30% Cr
7-10% Mo
Cr, Ni, Mo
Cr: oxide layer when dipped in Nitric acid (reduced corrosion)
Young’s Modulus
110 GPa
200 GPa
190 GPa
(used with cement)
Benefits
Yield strength; Ti > Stainless Steel
Stronger and more corrosion resistant than stainless steel; Excellent resistance to fatigue, cracking, and stress
Strong, cheap, relatively biocompatible; annealed, cold worked or cold forged; relatively ductile-contouring of plates and wires
Uses
Cementless joint replacements (total knee arthroplasty); Fracture fixation devices
Total joint arthroplasty (usually fixed with cement); Need to be inserted with a lower modulus polymer cement for fixation to prevent stress shielding of surrounding bone
Rarely used in new designs in joint replacement; Fracture fixation devices
Problems
Poor wear characteristics
*varies with smooth or porous surface
Co, Cr, Mo known to be toxic in ionic form; High modulus
Excessively corrosive in some cases Susceptible to fatigue cracking Very high modulus PMMA cement may cause fracture or tissue reaction

23. Composites Manufactured in several ways Young’s modulus = 200 GPa
Mechanical bonding between materials (matrix and filler)
Chemical bonding
Physical (true mechanical) bonding
Young’s modulus = 200 GPa
Benefits
Extreme variability in properties is possible
Problems
Matrix cracking
Debonding of fiber from matrix
Examples: concrete, fiberglass, laminates, bone

24. Ceramics Materials resulting from ionic bonding of Benefits Examples
A metallic ion and
A nonmetallic ion (usually oxygen)
Benefits
Very hard, strong, and good wear characteristics
High compressive strength
Ease of fabrication
Examples
Silicates , Metal Oxides - Al2O3, MgO
Carbides - diamond, graphite, pyrolized carbons
Ionic salts - NaCl, CsCl, ZnS

25. Ceramics (cont’d) Uses Problems Surface Replacement Joint Replacement
Very brittle & Low tensile strength
Undergo static fatigue
Very biocompatible
Difficult to process
High melting point
Expensive

26. Polyethylene Ultra high molecular weight (UHMWPE) High density
Molecular weight 2-6 million
Benefits
Superior wear characteristics
Low friction
Fibers included
Improve wear properties
Reduce creep
Used
Total joint arthoplasty

27. Bone Cement Used to fill gaps between bone and implant
Example: total hip replacement
If implant is not exactly the right size, gaps are filled regardless of bone quality

28. Bone Cement Polymethylmethacrylate
Mixed from powder polymer and liquid monomer
In vacuum
Reduce porosity
Increase strength
Catalyst (benzoyl peroxide) may be used
Benefits
Stable interface between metal and bone

29. Bone Cement (cont’d) Problems Inherently weak Exothermic reaction
Stronger in compression than tension
Weakest in shear
Exothermic reaction
May lead to bone necrosis
By handling improperly or less than optimally
Weaker
Extra care should be taken to
Keep debris out of the cement mantle (e.g., blood, fat)
Make uniform cement mantle of several mm
Minimize voids in the cement : mixing technique
Pressurize

30. Biodegradable materials
Fixation of horizontal maxillary osteotomies
Totally biodegradable self-reinforced polylactide (SRPLLA) plates
Pins
Poly-p-dioxanone (PDS)
Benefits
Gradual rate of absorption
Allows an optimal transfer of support to bone as it heals

31. Mechanical Properties of IM
As Implant materials have to function as bones, the mechanical properties of interest are
Elastic modulus
Ultimate tensile strength
They are listed in order of increasing modulus or strength
(in next 2 slides)

32. Elastic Modulus in increasing order of strength
Cancellous bone
Polyethylene
PMMA (bone cement)
Cortical bone
Titanium alloy
Stainless steel
Cobalt-chromium alloy

33. Ultimate Tensile Strength in increasing order of strength
Cancellous bone
Polyethylene
PMMA (bone cement)
Cortical bone
Stainless steel
Titanium alloy
Cobalt-chromium alloy

34. Young’s Modulus Interactive Figure

35. Additional Resources

36. The End

37. Fracture Blisters Blisters on swollen skin overlaying a fracture
Most often at tibia, ankle or elbow
Appear within hours of injury
Complicate or delay surgical treatment if present preceding care
No adverse affects if they appear following treatment

38. Varus Deformity at the knee
“A medial inclination of a distal bone of a joint from the midline”
Can occur at any joint; Knee shown
Before correction
After corrective implants C A B

39. Oteotomy Removal of a wedge of bone to correct a (varus) deformity
High Tibial Osteotomy

40. Proximal Femur (Hip) Fractures
Risk of fracture effected by
Age
Gender
Geographic location/ Ethnicity
Mental capacity
Bone strength
Pre-existing medical conditions